Is there a way to align multiple vertices to another vertex?
Set pivot point to 'active element' and transformation orientation to 'normal'.
Duplicate the edge you want them to be parallel to, move it with G and Z twice (along normal) and extrude it (E+Z) to create a clear normal.
Select your unstraight edge and afterwards the created polygon, so the polygon is the active element.
Hit S to scale, Z twice and move the mouse or just press 0 to make it completely straight. 
Delete the temporary polygon.




Works always, no matter how your object is weirdly rotated in 3D space.